Body temperature and cardiovascular regulation in older adults during 8‐hour exposures to ambient conditions experienced indoors during extreme heat events: A randomized crossover trial

Abstract

Most heat‐related deaths and hospitalizations occur in older adults, who spend the majority of their time in the home. However, recent studies aimed at improving our understanding of the physiological challenge posed by extreme heat events (EHE) have exposed participants to peak outdoor conditions, which differ markedly from those occurring indoors. In this randomized crossover trial (ClinicalTrials.gov identifier: NCT04348630), we assessed body temperature and cardiovascular responses in older adults exposed to a range of conditions commonly experienced indoors during EHE – from an air‐conditioned environment to a poorly insulated and ventilated domicile. We hypothesized that body core temperature and heart rate would be elevated in proportion to ambient temperature while mean arterial pressure would show the reverse response. We further surmised that cardiovascular adjustments to simple postural changes would be exacerbated at higher ambient temperatures.Fifteen older adults (5 women, 71 (SD 3) years, 25 (3) kg/m2) completed four 8‐hour exposures to 22°C (COOL), 26°C (TEMP), 31°C (WARM), and 36°C (HOT, each 45% relative humidity, <0.3 m/s air flow). Participants were seated throughout, except for during hour 7 when they completed a cardiovascular test battery including two standing from supine tests. Rectal temperature was monitored continuously. Heart rate and arterial blood pressures were measured every hour. The heart rate and systolic pressure responses to standing were calculated as standing – supine values. Resting responses at the end of the 8‐hour exposure and during the standing tests were evaluated by comparing linear mixed‐effect model estimated marginal means between adjacent conditions.Rectal temperature was elevated by 0.2°C [95% CI: 0.1, 0.4] in TEMP (37.1 (0.3)°C) compared to COOL (36.9 (0.4)°C), 0.5°C [0.3, 0.6] in WARM (37.6 (0.2)°C) compared to TEMP, and 0.3°C [0.2, 0.3] in HOT (37.8 (0.2)°C) compared to WARM (all P≤0.005). Heart rate increased 2 beats/min [0, 4], 7 beats/min [4, 10] and 5 beats/min [2, 7] from COOL (59 (8) beats/min) to TEMP (61 (6) beats/min), TEMP to WARM (69 (9) beats/min) and WARM to HOT (74 (11) beats/min), respectively (all P<0.001). Mean arterial pressure showed the reverse response, falling 7 mm Hg [2, 12] from COOL (101 (15) mm Hg) to TEMP (95 (11) mm Hg), 7 mm Hg [4, 9] from TEMP to WARM (88 (9) mm Hg) and 4 mm Hg from WARM to HOT (84 (10) mm Hg; all P≤0.02). While the change in heart rate from supine to standing did not differ between COOL and TEMP (P=0.68) or between WARM and HOT (P=0.46), it was 5 beats/min [1, 9] greater in WARM (14 (8) beats/min) compared to TEMP (10 [7] beats/min; P=0.006). The systolic response to standing did not differ between conditions (P=0.08).Body core temperature and cardiovascular responses in resting older adults were proportional to ambient temperature. We also show that the heart rate response when moving from supine to standing is exacerbated at ambient temperatures above 26°C. This study provides novel information on the physiological strain experienced indoors during EHE, both during rest and simple maneuvers consistent with activities of daily living.